Poultry manure is used as an organic fertilizer. Chicken droppings just after discharged from chicken or fowls contain water abundantly and have a characteristic odor. According to statistics for 1999, 843×104 tons of chicken droppings were discharged in poultry farms for egg production and 533×104 tons in poultry farms for raising broilers in Japan.
To utilize chicken droppings and excreta from other farm animals is important for the promotion of sustainable agriculture, for example in supplementing the farm land with organic matters and reducing the consumption of chemical fertilizers. On the other hand, from the viewpoint of preservation of the environment as well, appropriate methods of treating them are demanded.
Currently, chicken droppings are subjected, in the conventional manner, to drying treatment, fermentation treatment, carbonization treatment and incineration treatment at about 800° C. for ashing, and the calcined ash, after particle size adjustment, is used as a fertilizer.
In recent years, an increasing number of methods comprising burning chicken droppings to reduce the volume thereof and give an odorless ash, namely an calcined ash residue of chicken droppings, have been proposed as ecofriendly measures.
In an example, the chemical composition of the ash obtained from chicken droppings by 1.5 hours of treatment at 800° C. is as follows: CaO 32.0%, K2O 15.5%, P2O5 20.1%, SO3 10.0%, MgO 5.5%, Cl 5.5%, Na2O 3.0%, SiO2 3.0%, Al2O3 0.59%, Fe2O3 0.56%, MnO 0.21%, and ZnO 0.19%, with the loss on ignition (Ig. Loss) at 1000° C. being 3.7%.
In the above data, “%” indicates “% by mass” and, hereinafter, unless otherwise specified, “%” indicates “% by mass”.
Upon powder X-ray diffractometry, Ca3(PO4)2 [tricalcium phosphate],Ca5(PO4)3(OH) [hydroxyapatite], potassium chloride, quick lime (calcium oxide), slaked lime (calcium hydroxide) and calcium carbonate, among others, could be identified based on the chemical composition. Thus, the calcined ash residue of chicken droppings is rich in fertilizer nutrients, in particular phosphate(P2O5) and potash(K2O). However, they contain calcium compounds abundantly, which occur as quick lime formed upon incineration, slaked lime derived from quick lime as a result of absorption of moisture from the air during cooling and standing in the atmosphere, and undecomposed calcium carbonate. Thus, such calcined ash residues are strongly alkaline. Therefore, they, as such, cannot be mixed with other acid fertilizers or with ammonium nitrogen-containing fertilizers. The P2O5 component is mostly derived from hydroxyapatite and tricalcium phosphate. These phosphate salts have drawbacks: they are hardly soluble and low in fertilizer effect. The potassium component occurs as potassium chloride (KCl).
For efficient utilization of chicken droppings, a method has been proposed which comprises burning chicken droppings in the ambient atmosphere or in a low-oxygen condition to give an industrially useful fertilizer composition high in degree of elution of water-soluble potassium (cf. Patent Document 1: Japanese Laid-Open (Kokai) Publication No.2003-238277). This method indeed makes it possible to reduce the volume of chicken droppings, destroy the odor and remove hazardous organic compounds but cannot be expected to render the phosphate components effective or available.
It is known in the art to treat the calcined ash residue of chicken droppings with an acid to thereby render the phosphate components contained therein effective or available (cf. Patent Document 2: Japanese Laid-Open (Kokai) Publication No. S57-140387). For allowing the reaction between a powder and a liquid to proceed homogeneously and efficiently, it is generally recommended that both be subjected to reaction in the form of a slurry rich in water.
However, the resulting reaction product is high in water content and congeals or occurs as a paste; hence, as such, it cannot serve as a product.
A method of producing a phosphate, potash and magnesium-containing fertilizer has been proposed which comprises adding a small amount of thickening agent to the calcined ash, adding sulfuric acid and/or orthophosphoric acid and carrying out the reaction under hydrous conditions to convert the phosphates contained in the calcined ash to effective or available phosphate forms and increase the solubility of the potassium-and magnesium-containing components contained in the ash, and recovering the reaction product in the form of a powder to give the desired fertilizer, without grinding the same (cf. Patent Document 2).
What is important in this process is to obtain the product in the form of a powder by utilizing the heat generated by the reaction between the alkaline components contained in the calcined ash residue of chicken droppings and the acid to thereby raise the system inside temperature and evaporating the excess water. However, even by this method, the heat of reaction is insufficient to solubilize the phosphate components contained in the calcined ash residue of chicken droppings and evaporate the excess water, the temperature raising in the system inside is limited, a long reaction time is required, and the phosphate components are rendered effective or available only to an unsatisfactory extent.
Accordingly, it is a first object of the invention to provide a method of producing a novel phosphate and potash(PK)-containing compound fertilizer in the form of a powder with good handleability by efficiently carrying out the reaction between the calcined ash residue of chicken droppings and a mineral acid to render effective or available the hardly soluble Ca5(PO4)3(OH) [hydroxyapatite] and Ca3(PO4)2 [tricalcium phosphate] contained in the ash by converting them to citric acid-soluble components and, at the same time, maintaining the product in a neutral or slightly acidic condition to thereby make it possible to blend the product with another acid fertilizer and/or an ammoniacal nitrogen fertilizers.
A second object of the invention is to provide a novel phosphate and potash(PK)-containing compound fertilizer excellent in characteristics as produced by such a production method.